Metal die casting is a process in which molten metal is caused to flow into a cavity defined by a mold. In conventional metal die casting, molten metal is injected into the cavity. In semi-solid metal die casting processes, a metal billet is pre-heated to a point of softening, to a temperature above the solidus and below the liquidus to produce a partially solid, partially liquid consistency prior to placing the billet or "slug" in a shot sleeve in the casting machine.
Semi-solid metal die casting enables control of the microstructure of the finished part to a degree which produces a stronger part than is possible with conventional molten metal die-casting processes. As compared with conventional metal die-casting processes, semi-solid metal casting produces parts of improved casting quality in that they exhibit lower porosity, parts shrink less upon cooling enabling closer tolerances and physical properties are better. In addition, semi-solid metal casting has a reduced cycle time and the lower temperatures utilized result in decreased die wear. Because of the absence of molten metal there is less pollution and safety hazards are reduced.
In semi-solid metal die casting, a billet is first formed which is treated to form fine grained equiaxed crystals as opposed to a dentritic structure. Subsequent heating, forming and solidification of a formed part using a treated billet avoids the formation of a dentritic structure in the finished part.
To work successfully in semi-solid metal casting, the grain structure of a billet must exhibit the necessary degree of lubricity and viscosity to give good laminar flow in the die cavity. For example an untreated DC cast billet will shear along its dentritic axis rather than flow hence the need for fine grained equiaxed crystals.
Flowability is further affected by grain size and solid/liquid ratio. In addition forming parameters such as die temperatures and gate velocity will affect the casting process. Accordingly, all of the foregoing parameters have to be optimized in order to produce successful parts.
Metal forging is another process in which metal is caused to flow into a cavity defined by a mold. Unlike die casting, metal is not injected as a liquid into the cavity, but rather a solid billet or slug is placed between dies which are subsequently forced together to squeeze the billet or slug into the cavity as the die is closed. In semi-solid metal forging, the metal billet is pre-heated to a partially solid, partially liquid consistency prior to forging. The consistency is similar to that used for semi-solid metal die casting.
As in semi-solid metal die casting, the billet should consist of fine grained equiaxed crystals rather than a dendritic structure to optimize the flow of metal between the dies and to optimize the physical characteristics of the finished parts.
An earlier process for forming a treated billet involves the use of magnetic stirring during the cooling of a cast billet to break up and avoid the formation of a dentritic structure. Magnetic stirring is however a relatively slow and expensive process.
U.S. Pat. No. 4,415,374 (Young et al) describes an alternate process for forming a billet of aluminum for use in a semi-solid metal die casting process. Young et al describes a process having the following steps:
1. Melting and casting an ingot; PA1 2. Cooling the ingot to room temperature; PA1 3. Reheating the ingot above its recrystallization temperature but below its solidus temperature; PA1 4. Extruding the ingot; PA1 5. Cooling the ingot to room temperature; PA1 6. Cold working the ingot; PA1 7. Reheating the ingot above its solidus temperature; and PA1 8. Forming and quenching the ingot. PA1 i) heating the direct chill cast billet to a temperature above its recrystallization temperature and below its solidus temperature; PA1 ii) reducing the diameter of the heated billed from step i) and breaking down its grain structure by extruding it through an extruding die at said temperature above its recrystallization temperature and below its solidus temperature to form an extruded column without introducing any strain in addition to that associated with the extruding; PA1 iii) cutting the extruded column into billets; PA1 iv) heating a billet from step iii) to a thixotropic forming temperature; and, PA1 v) squeezing the heated billet from step iv) between the dies of a metal forming die set to form a part.
The ingot produced according to the process described in Young may then be subsequently heated to semi-solid casting temperature and formed into a part in a die casting process.
Even though Young avoids the requirement for magnetic stirring, it is nevertheless a cumbersome process including a large number of process steps.
More recently a process has been proposed in which a cast ingot is machined down to a billet of approximately one inch in diameter and deformed by subjection to a compressive force. The deformed billet is then heated to a temperature above its recrystallization temperature and below its solidus temperature. The billet is then cooled to room temperature for subsequent re-heating and use in a semi-solid metal casting process. This process however involves an expensive and wasteful machining operation and only appears to work with relatively small billet diameters of less than about one inch (approximately 25 mm) diameter.
It is therefore an object of the present invention to provide a process for semi-solid metal die casting which avoids not only magnetic stirring, but also eliminates many of the steps that would be required pursuant to the Young process.
It is a further object of the present invention to provide a semi-solid metal die casting process which avoids the machining cold working heating, cooling and re-heating steps associated with other processes.
It is yet a further object of the present invention to provide a process capable of forming billets for use in semi-solid metal die casting processes that may be significantly greater than about one inch (approximately 25 mm) in diameter.